.parent {
  display: flex;
  flex-wrap: wrap;/*可以换行*/
  width: 800px;
  border: 1px solid blue;
}

.child {
  /* //  flex: none | [ <'flex-grow'> <'flex-shrink'>? || <'flex-basis'> ] */
  /* //  If we don't want the items to stretch: */
  flex: 0 1 300px;
  /* //  If we do want the items to stretch: */
  /* flex: 1 1 300px; */
  
  /* //   etc. */
  border: 1px solid red;
  background: lightpink;
  font-size: 2rem;
  text-align: center;
}
